In the case of a High Voltage Direct Current (HVDC) transmission converter, a power semiconductor that can be turn-on/turn-off controlled is used in order to perform conversion between alternating current (AC) voltage and direct current (DC) voltage. Since the withstanding voltage of the power semiconductor is limitative, a plurality of semiconductor modules having power semiconductor circuits must be connected in series in order to perform high voltage processing. In order to configure the power semiconductor circuits, various semiconductor modules may be connected to each other.
As is well known, a well-known Modular Multilevel Converter (MMC) includes a plurality of sub-modules in which such power semiconductor circuits form two output terminals X1 and X2, and these plurality of sub-module is connected in series. Each of the sub-modules includes, for example, an energy storage unit and power semiconductors. The power semiconductors may be implemented using power semiconductor switches and freewheeling diodes, for example, IGBTs. This sub-module includes a so-called half-bridge or full-bridge circuit in which a plurality of power semiconductors is connected to each other. Furthermore, in the sub-modules of the MMC converter, one of the voltage of the energy storage units, zero voltage or the polarity-reversed voltage of the energy storage units appears at two output terminals.
In FIG. 1, a well-known MMC converter is shown. In this converter, one or more phase modules 1 are provided, and each of the phase modules 1 is configured such that a plurality of sub-modules 10 is connected in series to each other. As load connection terminals, AC voltage-side terminals L1, L2 and L3 may be connected to a three-phase load, for example, a three-phase AC power system.
In FIG. 2, an example of the equivalent circuit of such a sub-module 10 is shown. In the example of FIG. 2, the sub-module 10 includes a single half-bridge unit 11. The half-bridge unit 11 includes an energy storage unit 1 and a plurality of power semiconductors 6 connected in parallel to the energy storage unit 1. The power semiconductors 6 may be configured using power semiconductor switches 2 and 3 configured to be turn-on/turn-off controlled and freewheeling diodes 4 and 5. However, the sub-module 10 of FIG. 2 is problematic in that the sub-module 10 is damaged by high fault current.
In order to mitigate the above problem, a conventional method in which an auxiliary circuit unit was added between two half-bridge units, as shown in FIG. 3, was presented. In FIG. 3, another example of the equivalent circuit of the sub-module 10 is shown. In the sub-module 10 shown in the example of FIG. 3, two half-bridge units 12 and 13 are disposed on both sides and an auxiliary circuit unit 14 is added therebetween. In this case, the auxiliary circuit unit 14 includes one power semiconductor 7 and two diodes 8 and 9. By doing so, fault current is allowed to flow into the energy storage units 1 and 1′ of both half-bridge units 12 and 13, and thus the fault current is blocked or reduced. These flows of fault current in FIG. 3 are shown in FIGS. 4 and 5.
FIGS. 4 and 5 are diagrams showing the flows of fault current in the conventional sub-module. In FIG. 4, fault current from a system (in the direction of X1→X2) flows into the first energy storage unit 1 through the first power semiconductor 6 of the first half-bridge unit 12, and, at the same time, flows into the second energy storage unit 1′ through the diode 8 of the auxiliary circuit unit 14. Furthermore, in FIG. 5, fault current (in the direction of X2→X1) flows into the second energy storage unit 1′ through the second power semiconductor 6′ of the second half-bridge unit 13, and, at the same time, flows into the first energy storage unit 1 through the third power semiconductor 7 of the auxiliary circuit unit 14.
Meanwhile, recently, as research into such auxiliary circuit units has been carried out, there has been a demand for a technology that is capable of simplifying the configuration of an auxiliary circuit unit and reducing the manufacturing cost thereof while implementing performance and efficiency equal to or higher than those of the conventional technology.